The regulator said it is working with pipeline owner Legacy Oil and Gas Inc and the local government of Turner Valley, about 60 kilometers (37 miles) southwest of Calgary, to respond to the leak. The gas contains 1 percent hydrogen sulfide.

While no injuries have been reported due to the leak, hydrogen sulfide can cause serious injury or even death at levels as small as 100 parts per million. In smaller doses, it will cause irritation to eyes, nose and throat.

"The Town of Turner Valley has evacuated some residents as a precautionary measure. Legacy Oil and Gas has enacted its emergency response plan and is taking measures to manage the incident," the regulator said in a statement.

Turner Valley and many other municipalities in southern Alberta, including the city of Calgary, had already declared states of emergency on Thursday as heavy rains swelled rivers and streams and forced the closure of the TransCanada Highway, the country's main east-west route, near Canmore, 100 kilometers west of Calgary.

Legacy Oil and Gas said in a statement the leak had been caused by impacts from trees and other debris on a flow line in rising floodwater. The line is attached to its natural gas well, Royalite 19, in the Turner Valley.

The company said it was in the process of shutting down the line at the time due to the flooding.

"The Town of Turner Valley was already evacuating residents in the area of the leak in response to the flooding. As a precaution, Legacy worked with Turner Valley emergency services to evacuate additional residents in the area," Legacy oil said.

Legacy said its own air quality monitoring had not detected levels of hydrogen sulfide that would "exceed regulated levels for public health."

(Reporting by Scott Haggett in Calgary; Additional reporting by David Sheppard in New York and NR Sethuraman in Bangalore; Editing by Peter Galloway and Andre Grenon)